It is well known in the art to utilize filtration tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer or single layer arrangement of granular filter media such as sand, gravel and the like. In downward flow filtration systems of this type, water or other liquid containing suspended solid particles is introduced into the filtration tank from above, and clarified water is drawn off from a chamber either beneath or adjacent the individual filter cells. During downward flow through the individual cells, particulate matter is entrapped within the layer or layers of granular filter media, but eventually, the particulate matter clogs the filter media, thereby reducing the filtering capability of the system. Thus, there is a need for a periodic cleaning of the filter cells. It is also known to maintain such units in continuous operation during cleaning by the use of travelling bridge devices which move from one filter cell to the next, cleaning individual cells while permitting the filtration process to continue in the remaining cells.
Examples of travelling bridge apparatus of this type may be found in U.S. Pat. Nos. 5,089,117; 4,988,439; 4,957,631; 4,859,330; 4,764,288; 4,617,131; 4,540,487; 4,486,307; 4,133,766; 3,984,326; 2,235,227; and 2,302,449. Typically, travelling bridge systems include an overhead carriage, movable along tracks, guideways or the like, which carry a backwash hood which is either successively engageable with the open upper end of each filter cell or passes over the filter bed in close proximity to the media surface. For a downflow type filter, water or other treatment liquid is generally introduced by a backwash pump into the cell from below in a counterflow arrangement to the normal filtering direction. The backwash hood typically includes a suction head (i.e., a waste water pump) for drawing out fluid and debris forced to the surface of the filter cell as a result of the backwash. As the backwash of individual cells is completed, the travelling bridge moves the backwash hood to the next adjacent cell.
Typically, the filter media in each cell is supported above the bottom wall or floor of the tank by an underdrain structure which, in most existing designs, comprises fused porous polyethylene plates, or porous alumina oxide plates. The filter media is placed directly on the underdrain plates which, in turn, are supported on horizontal shelves on adjacent, vertical filter cell dividers. An L-shaped angle bracket or other retaining device is bolted through the cell dividers snugly on top of the underdrain plate to act as a means of capturing the underdrain plate. This is necessary because of the uplift exerted when flow is reversed through the underdrain plate by the backwash pump during a backwash cycle. There are, however, a number of problems with this type of underdrain:
(1) Fouling of the underdrain plates from a) micro-organisms growing in the pores and on the surface of the plates; b) oils and grease present in the feed flow; and c) suspended solids which penetrate the filter media during dosing or which are pushed into the porous plates in reverse during backwash. All of these eventually lead to irreversible fouling, resulting in poor filter performance and requiring underdrain replacement. Fouling occurs from these phenomena because the pore size openings in the underdrain are very small, primarily because the underdrain plates are manufactured by compressing and fusing beads together to form a 3/4 inch to 1 inch thick plates.
(2) Filter media loss is a common phenomena with existing porous underdrains because of the sealing methods used during installation. In the case of porous polyethylene plates, leakage at the end of the plate sections and at the point where the plates contact the basin walls is common because thermal expansion of the plates is quite high. Even regular expansion and contraction, due to temperature fluctuation, serve to push the sealing material out of the joints, allowing the filter media to leak into the underdrain plenum. Aluminum oxide underdrain plates are thermally stable, but are manufactured in 12 inch long sections which must be sealed at each joint with a flexible sealer. Compounding this problem is the fact that the plates are assembled with square butt joints, which make them even more difficult to seal. Moreover, underdrain sealer degradation compounds both of the above problems since the sealers have polyethylene bases and often soften over time due to submergence and chemical attack.
(3) Strength of the underdrain plates and their ability to support not only the weight of the filter media, but foot traffic during installation and uplift caused by the backwash pump as well, dictates a practical cell width of 12 inches. Thus, installation of the underdrain plates is critical to system operation due to the factors outlined under media loss. Workmen performing installation must be careful to properly seal the plates. Care must also be exercised to prevent the sealer from getting onto the surface of the underdrain to prevent blinding.
(4) Uneven distribution of backwash flow is an inherent problem with porous underdrains because of manufacturing tolerances which are typically + or - 25% permeability. Add to this the problems of fouling as described in item 1 above and the problem becomes significantly more acute.
Existing underdrain designs require distinct structural cell divider walls for two basic purposes: first, to provide a structural support for the underdrain plate as noted above, and second, to form a distinct sealed plenum for directing backwash water flow. In order to achieve these ends, it becomes necessary to seal the plenum and underdrain plates to prevent media loss, and to insure that backwash flow will pass upwards through the underdrain plate. Filter cell dividers must also be sealed to the base or floor and walls to prevent backwash water short circuiting, since the individual plenums are pressure zones during backwash. To effect such a seal, a special installation hardware is cast into or attached to the basin walls for installation of the cell dividers. This requires special forming of concrete basins which must be constructed to close tolerances so that the cell dividers fit correctly. In addition, leveling strips must be installed along the basin floor for levelling the cell dividers. The cell dividers must be anchored to these leveling strips, and then the entire floor area must be grouted with cement grout to seal the cell dividers to the floor.
While current design practices rely on the underdrain to provide uniform hydraulic distribution of both dosing and backwash flow, given the problems discussed above it is apparent that present underdrain systems are not completely satisfactory.
The proposed travelling bridge filtration system in accordance with this invention utilizes a new underdrain design including a plurality of pipe headers for dosing and backwash distribution as well as media support. Each pipe header is equipped with orifices or slots which are sized and spaced in accordance with hydraulic design and desired media grain size retention.
The underdrain pipe headers are not attached to the vertical cell dividers, but are self-supporting structures in and of themselves. Sealing to prevent media loss is not required as there are no seams or joints in the headers. Moreover, biological fouling is minimized since the orifice or slot size is substantially greater than the porous underdrains.
In addition, the underdrain pipe headers in accordance with this invention are much stronger than porous plates since they are fabricated from structural members. Installation is also simplified in that there are no hold-down angles to install nor joint sealers to apply.
In terms of performance, hydraulic distribution is superior to conventional porous plates since the design is based on standard engineering calculations, with machine placed orifices or slots and resultant machine precision tolerances. In addition, the reduced potential for fouling provides superior overall hydraulic characteristics.
Given a situation where a conventional porous underdrain and the pipe header underdrain of this invention were to foul, the ability to clean the underdrain is vastly improved with the pipe header of this invention. The porous underdrain, by reason of its formation from compressed fused beads, has a very tortuous path of flow through the thickness of the plate. When this path of flow becomes blocked, cleaning becomes difficult. On the other hand, pipe headers in accordance with this invention have a straight path through the wall of the pipe and can be cleaned by high pressure spray through the slots or orifices, or through the open ends of the headers in the adjacent backwash channel.
In accordance with the present invention, there is no need for distinct plenums as defined by individual cell dividers. Thus, cell dividers in accordance with this invention, if used, only serve to help direct the backwash flow. The sealing of the cell dividers to the floor or wall is not required, and no special brackets need be cast into the concrete basin walls. In addition, no leveling strips are required and no grouting or other sealing measures are necessary. Cell dividers, if used, can simply bolt in place since they are non-structural members. In accordance with an alternative and preferred arrangement, the cell dividers may be completely omitted from the present design. In other words, since cell dividers are utilized primarily to support the underdrain and to provide a plenum area for backwash, elimination of these functions by the proposed design also does away with the need for cell dividers per se.
In a first exemplary embodiment of the invention, a travelling bridge filtration unit is arranged as a single filter cell, with no cell dividers as found in conventional designs. An interior partition, extending longitudinally within the tank, serves to divide the tank into a filter basin area and a filtrate channel area. In addition, a unique underdrain system is provided which comprises a plurality of pipe headers arranged in parallel, between the interior partition and one side wall of the tank, transversely of the length of the tank. Each underdrain header is formed with a plurality of openings or slots along its length, arranged in a suitable pattern to insure uniformity of flow. Each header has a closed end remote from the interior partition, and an open end within the interior partition which is adapted to transfer filtered water to the filtrate channel during dosing, but which is also adapted to be engaged by the discharge of a backwash pump supported from the travelling bridge of the unit during backwashing. Filtration media substantially fills the cell from the bottom wall or floor of the tank, with the understanding that the underdrain manifolds or headers provide some degree of filter media support. The headers themselves are structural tubular members which are supported at opposite ends within the one side wall and the interior partition but otherwise require no additional support, thereby eliminating the need for individual cell dividers. In the exemplary embodiment, the travelling bridge supports a backwash hood movable along the length of the filter basin, in continuous movement or in stepwise fashion, to backwash successive portions of the filter bed. The hood supports a backwash removal header which serves to remove backwash material from the upper area of the filter bed, optionally assisted by an additional wash water pump.
Thus, in accordance with the broader aspects of the invention, there is provided in a travelling bridge filtration apparatus wherein a downflow filtration bed including one or more layers of filter media located in a tank is periodically backwashed by backwash means secured to a bridge adapted to travel across the top of the tank, the backwash means including a backwash pump, a backwash hood, and wherein the tank includes a filtrate channel for removing filtered effluent from the tank, an improvement comprising an underdrain comprised of a plurality of headers within the bed and in communication with the filtrate channel, each header provided with a plurality of apertures along its length.
Further details and advantages of the invention will become apparent from the detailed description which follows.